Field of the Invention
The invention relates to the field of optical signal transmission. In particular, the invention relates to a flat or sheet-like electrooptical component for sending and receiving electrical and optical signals. The invention also relates to a light-guide configuration containing such components for serial, bidirectional signal transmission, and to an optical printed circuit board.
Electrical printed circuit boards for driving electronic devices are commonplace in modern electronics. For many years, the speed with which the devices are operated has steadily increased. Modern processors already run at clock rates of above 1 GHz. Clock rates of several 100 MHz are aimed for and in some cases are already realized, even for comparatively slow memory chips.
As the speed of signal transmission increases in purely electrical printed circuit boards, difficulties increasingly occur. While it is possible in low-frequency operation at several MHz to realize, for example, a parallel serial bus concept without any problems, in high-frequency operation a range of problems arise.
For example, when using high frequencies with signal lines routed in parallel, the problem of crosstalk in which signal changes on one line induce interfering signals on neighboring lines increasingly occur. To remedy this, the lines must either be routed far apart from one another, which reduces the achievable data parallelism, or elaborate measures to shield neighboring lines from one another must be taken.
In the transmission of signals, distortions of the signal waveform also occur, in particular, in the case of signals traveling over relatively great distances with relatively long transit times, and it is necessary for this to be elaborately corrected or taken into account during the design of a circuit.
In the DRAM (Dynamic Random Access Memory) area, for example, so far there has been a reliance on purely electrical connections and terminals, since they can be electrically wired to printed circuit boards and to other components with good soldered bonds. With switching times of 1 to 5 ns, corresponding to 200 to 1000 MHz, however, high-frequency phenomena become noticeable, and can only be countered by good shielding and signal line reduction. A higher signal transmission rate consequently restricts the usable parallelism, a nuisance which has to be overcome to obtain further increases in overall performance.
Consequently, altogether considerable design and production effort has to be undertaken with electrical printed circuit boards to ensure interference-free and transit-time-adapted signal or data transmission at high signal frequencies.
To obviate these problems, optical connections have also been used. However, optical connections generally only take a unidirectional form between an electrooptical signal generator and an electrooptical signal receiver and then either do not allow read/write operation, or require two separate signal lines between the two end stations. Genuine bidirectional signal transmission between two stations that can in each case operate as a transmitter and receiver has until now required complex electrooptical circuitry.
If the transmitted signals are picked off serially at several points along the link, with all of the known methods this leads to a significant deterioration in the signal, so that repeated optical coupling out is only possible to a very restricted extent.
At the same time, the effective optical coupling in and out of light into and from an optical line is contrary to the requirements for simplest possible, interference-free bidirectional signal transmission. This is attributable to the wave character of the light and the associated directed, transversal electromagnetic signal propagation. Electrical signal transmissions on purely electrical printed circuit boards are unaffected by this problem, since electric current can be coupled into or out of a current conductor without great effort.
On the level of the contact pads, a purely optical solution has the disadvantage that the optical interface for bidirectional communication has to have both an input and an output, which with massive parallelism and simultaneous miniaturization of the components, leads to problems of space (known as pad-out).
A solution to the problem provided by components for signal multiplexing and processing, for instance in fiber-optic technology, requires high-quality components, which are consequently correspondingly complex to produce and are expensive. Bidirectional communication is then not possible for many applications on account of the complex structural form, or is not cost-effective on account of the associated costs.
Similarly, it is often not possible to achieve optical signal transmission with continuity, since optical signals are refreshed. In other words, a residual optical signal is converted into an electrical signal, amplified, and is optically re-emitted.